1. Field of the Invention
This invention relates to a cement retarder and a rapid hardening cement containing the same and more particularly to a cement retarder composed of three components and a rapid hardening cement containing such a retarder.
2. Prior Art
There have hereto been various proposals for a cement retarder used for a rapid hardening cement containing calcium sulfate and an amorphous calcium aluminate such as 12CaO.multidot.7Al.sub.2 O.sub.3 or 11CaO.multidot.7Al.sub.2 O.sub.3 .multidot.CaF.sub.2. Such retarders include, for example organic acids, borates, silicofluorides and the like. However, these retarders do not meet all the requirements called for as a cement retarder. The requirements are:
(1) that the handling time of a rapid hardening cement (i.e. a period of time starting from the time at which a rapid hardening cement is mixed with water to the setting time, which will be hereinafter referred to as H.T.) can be controlled within a wide range and practically this range should be from 10 to 60 minutes.
(2) that the H.T. of a rapid hardening cement should be in proportion to the amount of a cement retarder added.
(3) that a cement retarder shall not adversely affect the initial strength of a rapid hardening cement within the order of hours.
(4) that a cement retarder shall not cause pollution problems.
More specifically, borates and silicofluorides have good retarding effects. However, these compounds do not exhibit satisfactory initial strength and, to make matters worse, they are poisonous. Organic acids such as gluconic acid, citric acid, tartaric acid or malic acid are also inferior in the initial strength when solely used, whereas when used in combination with alkali carbonates this drawback can be avoided. Nevertheless, the requirement mentioned in item (2) above still remains unsolved, that is, the H.T. of the cement containing such retarders can not be controlled in proportion to the added amount of the retarders.